1. Field of the Invention
The present invention concerns a solid electrolyte type gaseous carbon dioxide sensor and, more in particular, it relates to an improvement of a humidity proof property for a detection electrode.
2. Description of the Prior Art
Solid electrolyte sensors under development at present usually comprise a detection electrode and a reference electrode disposed on both sides of a solid electrolyte as an ionic conductor.
Generally, in a case of detecting gaseous ingredients present in an atmosphere by using a solid electrolyte sensor, an ionic conductor in which specific ions are movable and, in combination with this specific ion conductor used as the solid electrolyte, a compound containing the specific ions and an aimed gaseous ingredient is used as a detection material being covered on an electrode, for example, made of platinum.
A gaseous carbon dioxide sensor based on such a principle uses, for example, a sodium ionic conductor such as of .beta.-alumina (general formula: Na.sub.2 O.nAl.sub.2 O.sub.3, n=5-11) or NASICON (general formula: Na.sub.1-x Zr.sub.2 P.sub.3-x Si.sub.x O.sub.12). In this case, a platinum gauge covered with sodium carbonate or the like is used as a detection electrode.
A typical reference electrode comprises gold or platinum alone or being covered with sodium carbonate or the like which is tightly sealed in air or gaseous carbon dioxide. Accordingly, while gaseous carbon dioxide as a gas to be measured can be in contact with the detection electrode but not with the reference electrode on the opposite side.
The sensor portion is heated upon operation usually to a constant temperature of about 400.degree. C. to 600.degree. C., in which an electromotive force of sodium ions is caused to the detection electrode corresponding to the partial pressure of the gaseous carbon dioxide in a gas to be detected which is in contact with the detection electrode and sodium ions in proportion with the difference of the electromotive force between both of the electrodes conduct through the ionic conductor. Accordingly, the concentration of the gaseous carbon dioxide can be detected by measuring the electromotive force.
However, in a case of the existent gaseous carbon dioxide sensor using sodium carbonate as the detection material for the detection electrode and using NASICON for the ionic conductor as described above, the characteristic of the electromotive force greatly suffers from the effect of a moisture content in the gas to be detected. FIG. 3 shows the electromotive force characteristic of the device, heated to an element temperature of 550.degree. C., relative to the change of concentration of gaseous carbon dioxide in air at a humidity of 20% and 75% as well as in an anhydrous state. As shown in the figure, if the air contains the moisture content even as low as 20%, the electromotive force relative to the change of the concentration of the gaseous carbon dioxide is reduced, failing to attain satisfactory detection and it can not be discriminated whether the change of the electromotive force is attributable to that of the concentration of the gaseous carbon dioxide or to that of the humidity.
In view of the above, although an improved method has been proposed, for example, of covering the detection electrode with such a gas permeable membrane that allows only the gaseous carbon dioxide but not water content to pass therethrough, preparation of the permeation membrane is extremely troublesome, as well as the effect is not sufficient and it has been difficult to dissolve the foregoing problem in this prior art.